The present invention relates to a lenticular lens sheet, a display front plate and a transmission type projection screen, all of which are suitable for projecting and observing an image supplied from an image source having a cell structure, such as a liquid crystal device (LCD), a digital micro-mirror device (DMD) or the like.
In the conventional art, there is a known rear projection type television which includes three colored (i.e., red, green, blue) cathode ray tubes (CRT) as the image source and a transmission type projection screen as the picture screen. For the above-mentioned transmission type projection screen, it has been required to diffuse image light in a wide range and decrease an influence of outside light.
FIG. 4 shows an example of the conventional transmission type projection screen. In order to meet the above-mentioned requirements, this transmission type projection screen comprises a lenticular lens sheet 40. The lenticular lens sheet 40 is provided, on an light incident face side 41 thereof, with a converging lens section 42, such as a lenticular lens. On a light emitting face 44 in the vicinity of a focus of the lens section 42, the lenticular lens sheet 40 includes a plurality of non-light emitting sections 47 covered with light absorbing layer 48 which will be referred to black stripes (BS), hereinafter. With the arrangement, the BS lenticular lens sheet 40 is capable of diffusing the light while decreasing the influence of the external light.
As the image source, a conventional art projection television has been developed with the above-mentioned LCD or DMD. Also in this projection television (TV), the above BS lenticular lens sheet has been employed for purposes of an improvement of diffusion performance and a prevention of the external light from reflection.
In the above-mentioned projection TV, however, there is a possibility of occurrence of moire due to the sampling effect of lenticular lens in case of projecting an image on the lenticular lens sheet having the above-mentioned cyclic structure repeated at regular pitches.
It is an established theory that, in order to prevent the occurrence of moire, a pitch of the lenticular lens is preferably decreased so as to be 1/3.5 (equal to ten thirty fifth) or less of a pitch of lattice pattern projected.
In addition, it should be noted that the projection TV having the LCD or DMD often produces a glaring picture which is called "scintillation". However, to decrease the pitch of the lenticular lens would be effective in order to weaken the above-mentioned scintillation.
Meanwhile, the transmission type projection screen having the BS lenticular lens sheet as shown in FIG. 4 operates to diffuse the light in a wide range of an angle of 40 degrees or more. Thus, in order to form the black stripes (BS) on the transmission type projection screen simultaneously, a distance between the incident lens and the light emitting face has to be established to be one point three times as large as a pitch of the incident lens. Under such a situation, in order to make the moire constituted by the lattice pattern projected on the screen and each pitch of the lens, inconspicuous, it is required to establish a lens pitch less than 0.4 mm and a lens thickness less than 0.54 mm.
However, if reducing the thickness of the screen as mentioned above, a rigidity of the screen will be deteriorated to get difficult to maintain the screen flat. In addition, it is very complicated to mold such a thin lens sheet with accuracy by an extruding mold method or the like.
Further, it has been executed that, due to the above-mentioned reason, the transmission type projection screen as the projection TV using the LCD or the DMD utilizes a lenticular lens sheet, which is provided only on a light emitting face side with colored lenticular lenses (light emitting side lenticular lens sheet), a lenticular lens sheet which is provided only on a light incident face side with colored lenticular lenses (light incident side lenticular lens sheet) or the like.
In the light emitting-side lenticular lens sheet some are shaped to have partially circular or partially elliptical cross sections, and others are shaped so as to utilize total reflection.
In the former lenses, there is a common problem that it is difficult to broaden a viewing angle since its lens angle relative to the projecting light exceeds the critical angle at respective foot portions thereby to cause the total reflection of the projecting light.
On the other hand, the later lens have a common problem of impossibility of an exact die transferal by the extruding process due to their singular configurations, so that they have no choice but to be produced by the casting process having a poor productivity.
FIG. 6 shows a relationship between an inclination angle at a light incident position of the light incident-side lenticular lens and the emission angle of the light. In FIG. 6, a letter .phi. designates a foot angle at the foot portion of the incident lens, a letter .theta. an emission angle of the light entering into the foot portion of the lens, an alphabet h a height of the incident lens, and l a distance between an incident point (i.e. the foot portion of the lens) and a converging point, respectively. Table 4 shows the emission angle and respective positions of the converging points relative to the lens angle at the foot portion of the lens. Note, in this table, an alphabet n denotes a refractive index and p a pitch of the lens.
TABLE 4 ______________________________________ emission angle relative to angle of foot portion of incident lens n = 1.5, p = 1.0 mm .phi. deg! .theta. deg! 1 mm! h mm! ______________________________________ 30 15.9 2.69 0.14 40 22.3 1.92 0.19 50 29.7 1.42 0.26 60 38.9 1.08 0.33 70 51.0 0.83 0.42 ______________________________________
In order to accomplish a wide view angle having more than the emission angle .theta. of 40 degrees in the light incident-side lenticular lens sheet, it is required to establish the lens angle .phi. more than 60 degrees at the foot portion of the lens, as shown in FIG. 6 and Table 4.
However, if increasing the lens angle simply, the external light entering into the lens through the light emitting face will be totally reflected on the incident lens portion, so that it projects from the light emitting face again together with the image light, as shown in FIG. 3B. In such a case, it is expected that the contrast of image is influenced remarkably and disadvantageously. It should be noted that, since most of the BS lenticular lens sheets have the light emitting faces which are disposed at general condensing points of their incident lenses, a distance between the incident lens portion and the light emitting face will be equal to a distance of (h+1). In Table 4, the distance (h+1) in case of 60 degrees of the lens angle at the foot portion of the lens .phi. is set to 1.41, while the distance (h+1) in case of 70 degrees of the lens angle .phi. is set to 1.25. Thus, it will be understood that, as mentioned before, the distance between the incident lens portion and the light emitting face must be established to be about 1.3 times as large as the lens pitch. This means that a reduction of a pitch of the lens makes a thickness of the lens thinner, thereby causing the rigidity of lens to be weakened and the forming of lens to be complicated.